There are already known various constructions of gas lasers, among them such having hollow dielectric waveguides. A particular advantage of the lasers of the latter type is that they are more compact than other gas lasers. Reducing the dimensions of the passage of the discharge housing provides increased gain, higher power generated per unit volume and an improved saturation parameter. This increase in power is achieved, as a result of the existence of well defined, low loss modes of laser propagation, despite the fact that linear gas waveguides leak radiation into the walls of the dielectric. The performance enhancements result from favorable de-excitation of the gas by wall collisions, from device operation at increased gas pressure, and from reduction in gas temperature due to the improved thermal conduction provided by the waveguide walls.
Recently developed configurations of waveguide gas lasers have produced a substantial increase in laser output power per unit volume in sealed-off (no gas flow or makeup) devices. One way of increasing the output from a gas laser device is to increase its effective length. However, the use of this technique in long life devices is limited due to the high intracavity flux levels incident on the resonator optics. So, for example, in a 100 watt Z-folded gas laser device, the peak flux density on the turning optics for a resonator employing a 25 percent output coupling mirror is at a level of about 30 kW/cm.sup.2.
One method by which one could substantially reduce this flux level is to increase the diameter of the waveguide all over its entire length. However, when this is done, there is experienced a loss in transverse mode discrimination, and transverse mode beating results. A 100 watt waveguide laser would also have a gain length of approximately 180 cm. The longitudinal mode spacing given by c/2 would be 83 MHz. Consequently, if the application of the laser required an offset in frequency from line center of approximately 41 MHz, longitudinal mode beating would occur, limiting the utility of the laser, especially in some coherent ladar applications.
An approach by which all of the above problems, namely the high intracavity flux density and the limited longitudinal mode spacing, can be circumvented is to use a phase-locked array of waveguide lasers. An early implementation of this approach is disclosed in a commonly assigned U.S. Pat. No. 4,688,228, in which a plurality of resonator cavities is arranged next to one another, with each adjacent two of such cavities being separated from each other only incompletely by a ridge which extends from one of the major internal surfaces bounding the cross-sectionally rectangular internal space of the laser body toward the other major surface, but which terminates short of such other major surface.
Experience with this waveguide gas laser arrangement has shown that it operated well when only two of such resonator cavities were provided. However, when the number of the resonator cavities that are arranged next to one another was increased to three or more, with all of the ridges still extending from one and the same major surface, problems were encountered with the quality of the combined laser beam emerging from the laser arrangement and particularly with phase locking between and among the resonator cavities. Such problems are attributable to the fact that a large open or unguided region exists at one of the major surfaces bounding the internal space subdivided by the ridges into individual resonator cavities. This open region not only permits radiation to leak from one of the resonator cavities to the adjacent one to achieve the desirable phase locking, but also, to a great disadvantage, brings about conditions in which higher-order transverse modes may and often will develop. As a consequence, a great number of transverse mode beats was observed in laser arrangements having several adjacent resonator cavities connected with one another by such a large open region.
Moreover, the gain and mode volumes of the previously proposed gas laser arrangements of this type, which are provided with cross-sectionally rectangular or U-shaped channels forming the resonator cavities, are not well matched to one another. This means that such channels include corners in which the gaseous lasing medium is being pumped and, as a result, exhibits gain. This has two disadvantageous consequences. First, the power dissipated in the corners so such cross-sectionally rectangular or U-shaped channels is wasted, resulting in a diminished efficiency of the gas laser arrangement. Secondly, gain within the corner regions can support modes other than the desired EH.sub.11 mode, resulting in mode beating in the outgoing laser beam and in further reduction in the useful power of such output laser beam.
Examples of gas laser cavity array arrangements which address at least the last-mentioned concern are disclosed in commonly assigned U.S. Pat. Nos. 4,807,232, 4,807,233 and 4,807,234, all issued on Feb. 21, 1989, as much of the disclosure of which as may be needed for supplementing the present invention, especially with respect to additional details and modifications that may be used in conjunction therewith, is incorporated herein by reference. Using the U.S. Pat. No. 4,807,233 as an example of a gas laser array arrangement in which the present invention may be employed to particular advantage, it is to be mentioned that, in this arrangement, the adjacent channels constituting the respective individual laser cavities are also separated from one another only incompletely by respective ridges so as to achieve the desired phase locking. This time, however, each of the ridges is constituted by a pair of projections each extending, in alignment with the other projection of the pair, from a different one of the major internal surfaces bounding the internal space of the laser body, and terminating short of the other projection to provide a gap through which the phase locking between the respective adjacent laser cavities takes place. In an attempt to match the gain volumes of the laser cavities to their respective mode volumes, these projections have been given identical generally cusp-shaped configurations, so that all of the individual laser cavities that are bounded thereby also have identical configurations, including identical cross sectional shapes.
Unfortunately, experience has shown that there exists a problem which limits the performance of the gas laser array not only of this type but also of other types, including those in which the phase locking between and among the laser cavities is achieved in a manner different from that used in the above patents, such as externally of the laser array proper, this problem being attributable to the fact that the temperature of the gaseous lasing medium is higher in the central region of the array than in the outermost regions. As a matter of fact, a temperature gradient is encountered in the transverse direction of the laser cavity array (along a main plane of the array or of an internal space of the laser body), with the temperature decreasing in each direction from a central plane of the array or internal space. This temperature gradient shifts or profiles the index of refraction transversely of the array. When such a temperature profiled array was operated with each of the laser cavities constituting an independent oscillator (not phase-locked) so as to be able to individually detect characteristic properties of the laser beams issuing from such cavities without being influenced by what was taking place in the other cavities, a spread in the operating frequencies was observed.
This spread in optical frequencies has a number of detrimental effects. First, if the individual resonator frequency is too large, phase locking with reasonable coupling is not achievable. Secondly, multiple modes may lase (mode beating). Thirdly, the output power level is degraded for staggered coupled devices of the type disclosed in the U.S. Pat. No. 4,807,232.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a waveguide gas laser arrangement which does not possess the disadvantages of the known arrangements of this type.
Still another object of the present invention is so to construct the arrangement of the type here under consideration as to provide low-loss phase coupling between and among the adjacent resonator cavities despite the existence of temperature differences or gradient in the gaseous lasing medium contained in such cavities.
A concomitant object of the present invention is so to design the above arrangement as to be relatively simple in construction, inexpensive to manufacture, easy to use, and reliable in operation nevertheless.